Detailed explanation of examples of Collections.sort sorting in Java

Comparator is an interface that can override the compare() and equals() methods. Next, this article will introduce you to Collections.sort sorting in Java. Friends who need it can refer to it

Comparator It is an interface that can override the two methods compare() and equals() for price comparison function; if it is null, the default order of elements is used, such as a, b, c, d, e, f, g. It's a, b, c, d, e, f, g. Of course, the numbers are also like this.

compare(a,b) method: Returns a negative integer, zero, or a positive integer depending on whether the first parameter is less than, equal to, or greater than the second parameter.

equals(obj) method: Returns true only if the specified object is also a Comparator and enforces the same ordering as this Comparator.

The second parameter of Collections.sort(list, new PriceComparator()); returns an int value, which is equivalent to a flag that tells the sort method in what order to sort the list.

The specific implementation code method is as follows:

Book entity class:

package com.tjcyjd.comparator; 
import java.text.DecimalFormat; 
import java.text.SimpleDateFormat; 
import java.util.GregorianCalendar; 
import java.util.Iterator; 
import java.util.TreeMap; 
/** 
 * 书实体类 
 * 
 * @author yjd 
 * 
 */ 
public class Book implements Comparable { // 定义名为Book的类，默认继承自Object类 
  public int id;// 编号 
  public String name;// 名称 
  public double price; // 价格 
  private String author;// 作者 
  public GregorianCalendar calendar;// 出版日期 
  public Book() { 
    this(0, "X", 0.0, new GregorianCalendar(), ""); 
  } 
  public Book(int id, String name, double price, GregorianCalendar calender, 
      String author) { 
    this.id = id; 
    this.name = name; 
    this.price = price; 
    this.calendar = calender; 
    this.author = author; 
  } 
  // 重写继承自父类Object的方法，满足Book类信息描述的要求 
  public String toString() { 
    String showStr = id + "\t" + name; // 定义显示类信息的字符串 
    DecimalFormat formatPrice = new DecimalFormat("0.00");// 格式化价格到小数点后两位 
    showStr += "\t" + formatPrice.format(price);// 格式化价格 
    showStr += "\t" + author; 
    SimpleDateFormat formatDate = new SimpleDateFormat("yyyy年MM月dd日"); 
    showStr += "\t" + formatDate.format(calendar.getTime()); // 格式化时间 
    return showStr; // 返回类信息字符串 
  } 
  public int compareTo(Object obj) {// Comparable接口中的方法 
    Book b = (Book) obj; 
    return this.id - b.id; // 按书的id比较大小，用于默认排序 
  } 
  public static void main(String[] args) { 
    Book b1 = new Book(10000, "红楼梦", 150.86, new GregorianCalendar(2009, 
        01, 25), "曹雪芹、高鄂"); 
    Book b2 = new Book(10001, "三国演义", 99.68, new GregorianCalendar(2008, 7, 
        8), "罗贯中 "); 
    Book b3 = new Book(10002, "水浒传", 100.8, new GregorianCalendar(2009, 6, 
        28), "施耐庵 "); 
    Book b4 = new Book(10003, "西游记", 120.8, new GregorianCalendar(2011, 6, 
        8), "吴承恩"); 
    Book b5 = new Book(10004, "天龙八部", 10.4, new GregorianCalendar(2011, 9, 
        23), "搜狐"); 
    TreeMap tm = new TreeMap(); 
    tm.put(b1, new Integer(255)); 
    tm.put(b2, new Integer(122)); 
    tm.put(b3, new Integer(688)); 
    tm.put(b4, new Integer(453)); 
    tm.put(b5, new Integer(40)); 
    Iterator it = tm.keySet().iterator(); 
    Object key = null, value = null; 
    Book bb = null; 
    while (it.hasNext()) { 
      key = it.next(); 
      bb = (Book) key; 
      value = tm.get(key); 
      System.out.println(bb.toString() + "\t库存：" + tm.get(key)); 
    } 
  } 
}

Custom comparator and test class:

package com.tjcyjd.comparator; 
import java.util.ArrayList; 
import java.util.Collections; 
import java.util.Comparator; 
import java.util.GregorianCalendar; 
import java.util.Iterator; 
import java.util.List; 
public class UseComparator { 
  public static void main(String args[]) { 
    List<Book> list = new ArrayList<Book>(); // 数组序列 
    Book b1 = new Book(10000, "红楼梦", 150.86, new GregorianCalendar(2009, 
        01, 25), "曹雪芹、高鄂"); 
    Book b2 = new Book(10001, "三国演义", 99.68, new GregorianCalendar(2008, 7, 
        8), "罗贯中 "); 
    Book b3 = new Book(10002, "水浒传", 100.8, new GregorianCalendar(2009, 6, 
        28), "施耐庵 "); 
    Book b4 = new Book(10003, "西游记", 120.8, new GregorianCalendar(2011, 6, 
        8), "吴承恩"); 
    Book b5 = new Book(10004, "天龙八部", 10.4, new GregorianCalendar(2011, 9, 
        23), "搜狐"); 
    list.add(b1); 
    list.add(b2); 
    list.add(b3); 
    list.add(b4); 
    list.add(b5); 
    // Collections.sort(list); //没有默认比较器，不能排序 
    System.out.println("数组序列中的元素:"); 
    myprint(list); 
    Collections.sort(list, new PriceComparator()); // 根据价格排序 
    System.out.println("按书的价格排序:"); 
    myprint(list); 
    Collections.sort(list, new CalendarComparator()); // 根据时间排序 
    System.out.println("按书的出版时间排序:"); 
    myprint(list); 
  } 
  // 自定义方法：分行打印输出list中的元素 
  public static void myprint(List<Book> list) { 
    Iterator it = list.iterator(); // 得到迭代器，用于遍历list中的所有元素 
    while (it.hasNext()) {// 如果迭代器中有元素，则返回true 
      System.out.println("\t" + it.next());// 显示该元素 
    } 
  } 
  // 自定义比较器：按书的价格排序 
  static class PriceComparator implements Comparator { 
    public int compare(Object object1, Object object2) {// 实现接口中的方法 
      Book p1 = (Book) object1; // 强制转换 
      Book p2 = (Book) object2; 
      return new Double(p1.price).compareTo(new Double(p2.price)); 
    } 
  } 
  // 自定义比较器：按书出版时间来排序 
  static class CalendarComparator implements Comparator { 
    public int compare(Object object1, Object object2) {// 实现接口中的方法 
      Book p1 = (Book) object1; // 强制转换 
      Book p2 = (Book) object2; 
      return p2.calendar.compareTo(p1.calendar); 
    } 
  } 
}

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